Conductive wire is often used to transmit electronic signals. The wire must be protected, or insulated, and thermoplastic coatings extruded from a molten state onto and around the wire are ordinarily used for this purpose. The thermoplastic materials are selected on the basis of several criteria including high dielectric strength, low dielectric constant, and low dissipation factor. It has previously been found that if the thermoplastic material is foamed as it is applied to the wire, the dielectric constant is desirably reduced due to the formation of numerous small non-interconnected cells in the foam. U.S. Pat. No. 3,072,583 discloses a nucleated foaming process for extruding perfluorinated polymer foam around transmission wire with a dissolved gas blowing agent. It is also known to foam molten resin with gas either injected under pressure or released by chemical agents incorporated in the resin. Foamed insulation using other thermoplastic materials, especially polyolefins, is also widely known.
Foams are also useful in applications other than wire insulation. Examples include foamed sheet for electrical insulation, thermal insulation, and cushioning; foamed pipe and tubing; and injection molded pieces.
A nucleating agent is needed in the foaming process to achieve a uniform, small cell structure. A preferred art-recognized nucleating agent for fluoropolymer resins is boron nitride (BN), an inert white ceramic powder. A 0.5-2.0 wt % loading of BN when used alone is normally used to provide foam cell nucleation.
U.S. Pat. No. 4,764,538 discloses synergistic combinations of BN and certain inorganic salts which give greatly enhanced foam nucleation in fluoropolymers. This combination not only allows a significant reduction in the amount of BN needed but also gives improved foam in terms of smaller cells. Improved capacitance uniformity and dimensional uniformity are also attained.
U.S. Pat. No. 4,877,815 discloses sulfonic and phosphonic acids, and salts of the acids, which give very efficient foam cell nucleation in a wide variety of thermoplastic materials at low concentrations. Additionally, as disclosed therein, these acids and salts are beneficially used in conjunction with BN. As additionally disclosed therein, these acids and salts are also beneficially used in conjunction with BN and calcium tetraborate together, i.e., a combination covered by U.S. Pat. No. 4,764,538.
U.S. Pat. No. 3,072,583 states a preference for BN having an average particle size of less than 10 .mu.m. U.S. Pat. Nos. 4,764,538 and 5,023,279 characterize the BN used therein as having surface areas, respectively, of about 8 m.sup.2 /g and 8.6 m.sup.2 /g. Japanese patent application publication 59-011340 (1984) discloses that BN is most effective as a blowing coagent (foam nucleating agent) for fluorinated resin if BET surface area is 5-10 m.sup.2 /g.
Various grades of BN heretofore used as foam nucleating agents are available from manufacturers such a Carborundum and Advanced Ceramics. It is believed that such BN grades are produced by processes that involve comminuting relatively large crystals and aggregates of crystals that result from crystal growth, to obtain powders having desired average particle size. Electron micrographs of powder samples show that such particles are generally irregular, with jagged edges, and indicate the presence of very small particles. See FIG. 2.
Further improvement in foaming efficiency, e.g., reduction in foam cell size and improvements in capacitance uniformity and dimensional uniformity, are desired. This is especially so for smaller wire constructions of interest in the electronics field, so that, for example, foam cell dimensions will be small with respect to the radial dimension of the thin insulation.